support for assertEquals([String message, ] CharSequence, CharSequence)

This contribution fixes a problem I've run into when using assertEquals on two CharSequence.

It basically already works, of course - BUT one is left wondering why e.g. the Eclipse (and presumably other IDE as well) JUnit view's Result Comparison pop-up does not work on double-clicking a failure.. this is because, currently, assertEquals on two CharSequence throws only a generic AssertionFailedError and not the ComparisonFailure expected by such UIs.

PS: As the pom.xml specifies jdk.version 1.5 and <compilerVersion>1.5</compilerVersion>,  I was assuming, hopefully correctly, that introducing a @since 1.4 CharSequence was not a problem for backward compat.
We have not been making changes to the code under `junit.framework`